Well I've now got the answer and it confirms my suspicion that such an
arrangement would be inadvisable :-
# 70 to 80 °C (158 to 176 °F) - Disinfection range
# At 66 °C (151 °F) - Legionellae die within 2 minutes
# At 60 °C (140 °F) - Legionellae die within 32 minutes
# At 55 °C (131 °F) - Legionellae die within 5 to 6 hours
# 50 to 55 °C (122 to 131 °F) - They can survive but do not multiply
# 20 to 50 °C (68 to 122 °F)- Legionellae growth range
# 35 to 46 °C (95 to 115 °F) - Ideal growth range
# Below 20 °C (68 °F) - Legionellae can survive but are dormant
It is interesting that building regs in Scotland now require a TMV for
baths in new builds but categorically state that the TMV must be as
close to the outlet as possible and that the tank water temperature
must be in excess of 60C.
I thought both thermal stores & heat banks use heat exchangers and the only
thing that comes out of a hot tap is "mains" water. Some of this water
could have sat in an internal heat exchanger for some time but that's no
different to the hot water reservoirs on combi boilers.
There are none.
Legionalla seems to have become noticed about the same time
people started trying to keep their water supplies sterile.
This might be due to the resulting loss of natural immunity
to the bacteria, which has likely been around for ages (it's
naturally present in rain in small quantities). When a person
who has lost their immunity through lack of exposure at home is
then exposed to a non-sterile supply (often in hospital or other
institution), they are much more vulnerable to pick it up.
If this is one of the main causes of infection (I'm not aware
of any research on this, but the hypothesis fits the observations),
it explains why you can't catch it at home -- you either have a
contaminated supply and are hence immune, or you have a sterile
[email address is not usable -- followup in the newsgroup]
>I posted a query some time back about the advisability of having a
>heatbank set up with a TMV to control the DHW to something like 50C as
>suggested in the DIYWiki. My concern was that distributing water at
>this temperature could be a spawning ground for legionella but was
>querying if anyone knew anything about this - there was no answer !!
Didn't notice the question.
In large buildings it has been recommended for decades that domestic
hot water is distributed round the building at high temperature and
mixed down to a lower temperature at the fitting.
However, most houses are not large buildings and the extra
complications of such an approach are unlikely to improve safety
much. For example large buildings tend to have hot water being
circulated constantly in the hot water mains, which gives time for
the little nasties to breed nicely. Few houses have such a setup.
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
I think I had answered it at the time, or a very similar query.
Store at 60 degC, mix at the outlet, as recommended in the HSE's L8
document, which has been the commercial code of practice and guidance
in the UK for some years. I think the range of temperatures above are
from L8 but I can't be bothered to check.
It's nice you've managed to verify what (I think) I'd said, shame it
took so long.
I don't think the figures are collected by the HSE because the
relevant legislation is the Health & Safety at Work Acts, which don't
apply to domestic circumstances. Commercial building owners and
managers have a 'duty of care' defined in law; householders don't.
I think it probably happens a lot (judging by the unbelievably
hideous state of many domestic systems that I've seen), but no-one
goes to the effort of sampling domestic water systems to verify the
source of one infection. There may be numbers on the HSE website; I
suspect they get lumped in with the bulk of 'source unknown'
The only domestic case I know of concerned an American plumber who had
a second home or cabin. He fired up the heating and had a shower
shortly after arriving there for the first visit in several months.
Afetr he became ill, he was being treated for a chest infection but
wasn't tested for legionella until the penny eventually dropped and he
realized he'd done a silly thing.
> The only domestic case I know of concerned an American plumber who had
> a second home..............
This one; not quite as I had remembered.
># 20 to 50 °C (68 to 122 °F)- Legionellae growth range
># 35 to 46 °C (95 to 115 °F) - Ideal growth range
To be complete this information would need to include how quickly
they multiply. Minutes, hours or days.
Time information is given for the higher temperatures.
> On Wed, 31 Oct 2007 15:06:55 -0700 someone who may be robgraham
> <robkg...@btinternet.com> wrote this:-
>># 20 to 50 °C (68 to 122 °F)- Legionellae growth range
>># 35 to 46 °C (95 to 115 °F) - Ideal growth range
> To be complete this information would need to include how quickly
> they multiply. Minutes, hours or days.
> Time information is given for the higher temperatures.
I find the information I've read on Legionella rather unconvincing.
Why is it not raging in the cold water system
in countries where the ambient temperature is in the "Ideal growth range"?
Is it in fact commonly found in domestic hot water systems?
That must be easy enough to determine.
If so, I would have thought it would be easy enough
to kill the bacteria.
ISTR that transmission to humans only occurs by inhalation - ie
In a heat bank the incoming cold fresh water would be taken to over 60C,
killing the legionella bacteria, and then down to a now safe 50C. Mains
water rarely has the bacteria in dangerous form. It gathers while standing
in tanks at the correct temperature.
In short, you have nothing to fear with a heat bank. It is storing water at
50C which is the problem. The advice for "stored water", (heat banks to not
store water that is ion contact with people), is store over 60C and then
blend down with a TMV to the desired temperature. This in effect will mean
having a smaller cylinder as more energy is stored in the same water volume.
He could have contracted that in he hotel he was kin and it came out in his
second home, with the home having nothing to do with it. He is assuming.
>> This one; not quite as I had remembered.
> He could have contracted that in he hotel he was kin and it came out in
> second home, with the home having nothing to do with it. He is assuming.
I didn't think it was completely obvious that he had had Legionella.
As I understood the story (which I didn't read very carefully)
he had pneumonia, and after this was cured he showed immunity to Legionella.
According to my reading, 15% (IIRC) of the US population show this immunity,
so it would not be clear that he had just had the disease,
which is a kind of pneumonia, as I understand it.
His symptoms did not seem to follow those of Legionella,
eg he felt ill immediately after taking a shower,
while Legionella is said to take a minimum of 2 days to take effect.
That is why I assumed the hotel was the place.
They give the information for the time taken to kill legionella. They
don't give the time taken for the bacteria to, say, double, because it
also depends on the nutrients (limescale, tallow, leather washers,
hemp; vegetable, animal or insect remains from unhygenic cold water
storage tanks, etc..).
The time can be misleading. 60 degC wil kill the bacteria, but it
usually manages to survive under limescale accumulations in the bottom
(coolest bit) of water storage cylinders, or under bio-films on
uninsulated pipework. It then re-infests the system.
He was slightly ill before he got in the shower.
It doesn't say how long after his shower that he was diagnosed with
It follows the established pattern; compromised immune system (NHS
hospital patients, elderly American legion veterans); stagnant, tepid
water; inhalation of an atomized of water spray. The shower remains
the prime suspect, IMHO. You could pretty much guarantee it was
It might have been the hotel. I'd be surprised, commercial maintenance
standards are usually higher, given the threat of litigation.
An Italian study seems to clear them:
>From the first one it appears the location of a TMV in a domestic hot
water system wouldn't make much difference, it's down to the type of
water heater and how it's used.
Instead of partially heating a big hot water cylinder, maybe it's best
to have... two hot water cylinders! :)
No, it's two combis.
Matt, you do surprise me, as you are actually learning.
>I find the information I've read on Legionella rather unconvincing.
I don't (and in a previous existence my staff tested water systems
regularly for the little nasties). It can be a health problem and
people are no longer with us as a result.
What I do not like is panic reactions from people who don't know
much about the subject (I'm not suggesting you are in this category)
who imagine that everyone is about to die. As with asbestos this is
a risk which can be minimised by good engineering practice. While
the risk is not zero it is low enough to be reasonable with suitable
The same is true of a number of systems. Where I once worked we lost
two men who opened up a pressurised heating system before it had
been depressurised. The result was inevitable, they died a horrible
death and those who entered the space to try and rescue them and
then removed the bodies are still haunted by it. A distant colleague
lost an electrician who had got into the habit of opening up the
panels of high voltage equipment before it had been earthed. It was
only 11kV. It appears he thought this was safe as long as he didn't
do any more than remove the panels, but one day he dropped his
spanner onto the still live busbars. I can still recall the day the
head engineer reminded us that we (the organisation concerned) had
just killed a family in a house. The fan on a boiler had failed and
it had not locked itself out. That taught me about accepting
responsibility for something which was not directly one's fault.
None of these events mean that the items concerned are unacceptably
unsafe, only that safe systems of work and fail-safe equipment are
part of risk management.